▎ 摘　　要

Deposition of hexagonal boron nitride (h-BN) over large areas is an essential requirement for scaling electronic applications of 2D materials. Through an understanding of the physical chemistry of the popular ammonia-borane route, this work demonstrates control that can yield a range of deposits from isolated single crystal islands to polycrystalline monolayers with 15 micron islands to bulk multilayers. By constraining supersaturation both at the source and in the reactor, a five-orders-of-magnitude reduction in the unwanted nanocrystalline boron nitride (n-BN) density per mu m(2) has been achieved. Clean layers over 6 inch square areas and 4 inch wafer scale transfers prove scalability. The state-of-the-art mobility, 28 000 cm(2) v(-1) s(-1), achieved by graphene transistors synthesized on these h-BN layers is a proof of their having met the substrate requirements of 2D electronics.